1. Field of the Invention
The present invention relates to a pneumatic servo valve, and more particularly, to a pneumatic servo valve, wherein displacement of a spool due to flow force of a fluid is prevented by firmly holding the spool just after position control for the spool is completed, thereby ensuring the stability of a valve system, and an encoder is employed as a spool position detecting means.
2. Description of the Prior Art
A pneumatic servo valve is a type of valve for diverting a flow passage of air and controlling a flow rate using axial movement of a spool by a servo solenoid.
Generally, such a pneumatic servo valve is of a multi-port type having a 3- or 5-way configuration. Driving signals of a DC 24 Volts and control signals of 0 to 10 Volts are employed in the pneumatic servo valve. Since the spool is positioned at a neutral point when there is no control signal or the voltage of the control signals is 5 Volts, a fluid introduced into a supply port cannot flow. When the voltage of the control signals is an arbitrary value between 0 and 5 Volts, or 5 and 10 Volts, however, the spool moves in proportion to the voltage of the control signals. As the spool moves, the supply port through which the fluid is supplied communicates with any one of discharge ports to define a flow passage.
A servo solenoid for controlling the position of the spool is a solenoid in which a plunger (spool) moves by means of magnetic force and the position of the plunger is fedback to compensate errors again. At this time, a Hall sensor is used as a feedback sensor, wherein the position of the plunger is detected using changes in the magnetic force depending on movement of a permanent magnet attached to the plunger.
Such a conventional pneumatic servo valve will be described in detail with reference to FIG. 1.
As shown in FIG. 1, the conventional pneumatic servo valve comprises a main body 1 which has a supply port 2 at an upper portion thereof, A and B exhaust ports 3a and 3b on both sides of the supply port, and A and B discharge ports 4a and 4b provided at a lower portion thereof to selectively communicate with the supply port 2 or the A and B exhaust ports 3a and 3b; a sleeve 5 which is fixedly installed in a hollow portion 1a of the main body 1 and has slots 5a communicating with the supply port 2 and the discharge ports 4a and 4b; a spool 6 axially slidably installed in the sleeve to control the flow of a fluid according to the position thereof; a torque motor 7 installed at one side of the main body 1 to control the driving of the spool in response to electrical signals; and a Hall sensor 8 for sensing changes in magnetic force due to movement of a permanent magnet 7a installed in the spool 6 to detect the position of the spool 6.
As for an operational principle of the pneumatic servo valve, a forward or rearward sliding distance of the spool 6 is controlled according to the polarity and intensity of an electric current applied to the torque motor 7. That is, if a central valve 6a of the spool 6 moves rightward when viewed in the figure, the supply port 2 communicates with the A exhaust port 3a through a reduced diameter portion 6b of the spool to define a flow passage. On the contrary, if the central valve 6a of the spool 6 moves leftward when viewed in the figure, the supply port 2 communicates with the B exhaust port 3b through the reduced diameter portion 6b of the spool to define a flow passage.
Meanwhile, if a left valve 6c of the spool 6 moves leftward when viewed in the figure, the A discharge port 4a communicates with the A exhaust port 3a through the reduced diameter portion 6b of the spool 6 to define a flow passage therebetween. If a right valve 6d of the spool 6 moves rightward when viewed in the figure, the B discharge port 4b communicates with the B exhaust port 3b through the reduced diameter portion of the spool to define a flow passage therebetween.
In such a conventional pneumatic servo valve, abnormal displacement (axial displacement) of the spool may occur due to large or small flow force of a fluid. In this case, as shown in FIG. 2, the central valve 6a is abnormally shifted in a direction designated by an arrow. Thus, the area of the slot 5a opened or closed is not constant, resulting in changes in a flow rate of the fluid. Accordingly, it is difficult for the servo valve to accurately control operations of a working body.
For example, assuming that such a pneumatic servo valve operates a pneumatic cylinder (working body), changes in a flow rate in the servo valve cause a flow rate supplied into the cylinder to be inconstant, resulting in abnormal displacement of a rod of the cylinder. Thus, it is difficult to accurately (or precisely) control the operation of the cylinder.